text: Any pair of symmetry-related X-C-C-Y fragments (where X and Y are different magnetically active nuclei) as well as XYC-CXY (cis or trans) and XC-CY fragments may show magnetic inequivalence when the heteronuclear coupling constants (J or J) are non-negligible. In principle, the magnetically active nuclei may also be disposed on non-carbon atoms.
  A classic example is the H-NMR spectrum of 1,1-difluoroethylene. The single H-NMR signal is made complex by the J and two different J splittings. The F-NMR spectrum will look identical. The other two difluoroethylene isomers give similarly complex spectra.